This invention generally relates to abrasive blasting systems, and more particularly to a vacuum entrance device or density controller for vacuuming high density abrasive blast media, such as steel grit.
Steel grit has been used in abrasive blasting operations, such as for removing paint and debris from buildings, bridges, or other structures. Steel grit is superior to sand in use during such abrasive blasting operations because sand typically becomes pulverized during low velocity abrasive blasting operations. Steel grit, however, does not become pulverized, even during high velocity abrasive blasting operations. Therefore, steel grit may be used for sustained high velocity abrasive blasting procedures thereby permitting significant reductions in the time required for an abrasive blasting process. However, despite its laudable characteristics, steel grit is significantly more dense than sand. Typically, steel grit weighs 260 lbs./cubic ft., however; sand weighs less than 100 lbs./cubic ft.
Vacuum generators have been used to convey collected solid particles and abrasive media during an abrasive blasting operation. Vacuum entrance devices (also called density controllers) have been designed for use with such vacuum generators for mixing the solid particles and abrasive media within a conveying airstream. Although these density controllers have operated with varying degrees of success with low density abrasive media, such as sand for example, conventional density controllers are unable to effectively accommodate high density abrasive media, such as steel grit.
When used with steel grit,conventional density controllers create large fluctuations in system air conveying velocities, create large fluctuations in system vacuum pressure, require extensive complex adjustments, and cause continuous choking of the steel grit and solid particles within the vacuum generation system. Choking occurs when an excessive volume of high density steel grit is permitted to enter a conveying pipe of the vacuum system. When choking occurs, the steel grit falls out of the vacuum airstream and clogs the conveying pipe.
Another problem associated with the use of steel grit in abrasive blasting systems relates to the high conveying velocity required to effectively move the steel grit through the blasting system without clogging the system. FIG. 4 is a graph showing the associated required conveying velocities for different size and weight materials. As shown on the graph, generally, the lighter the material the lower the required conveying velocity, and the heavier the material the greater the required conveying velocity. For example, the conveying velocity of average size steel grit weighing 260 lbs./cubic ft. ranges from 10,000 ft./min. to 11,000 ft./min. depending on particle size. In order to convey the particles with the required velocity and thus not clog the system, it is necessary to provide and maintain sufficient system vacuum and airflow. If sufficient vacuum and airflow are not maintained, the grit will not be drawn into the system and conveyed through the system with the required velocity and the system will become clogged.
It is easier to provide and maintain sufficient airflow for conveying material lighter than steel grit than it is to convey heavy steel grit since the required conveying velocities for lighter materials are significantly lower than the conveying velocity for steel grit. Thus, the system vacuum and airflow required to draw and convey a lighter material are easier to produce and maintain than the vacuum and airflow required to draw and convey steel grit.
As a matter of comparison, as indicated in Fan Engineering, eighth edition, published by Buffalo Forge Company, the conveying velocity of corn is 5600 ft./min., of oats is 4500 ft./min., of sand is 7000 ft./min. and of wheat is 5800 ft./min. These light materials are easier to convey and may be conveyed in varying volumes by adjusting the size and shape of the material inlet ports.
In order to ensure the required system vacuum and airflow for conveying steel grit are supplied, it is necessary to maintain the end of the nozzle a fixed distance from the end of the density controller. If the nozzle end is too close to the controller end, too much steel grit will be drawn into the system and the system will clog. If the nozzle end is too far from the controller end, the nozzle will not provide sufficient airflow to keep the grit suspended in the system and the system will become clogged. Typical systems use fasteners such as screws, bolts, or the like to maintain a fixed distance between the end of the density density controller and the nozzle end. However, over time as the nozzle/controller combination is repeatedly inserted into the dense, heavy steel grit, the fasteners may be knocked loose and, as a result, the density controller will become displaced relative to the nozzle, thereby negatively affecting the operation of the abrasive blasting system.
Also, machine operators may loosen the fasteners and manually reposition the density controller relative to the nozzle, thereby negatively affect the efficiency and operation of the blasting system.
The foregoing illustrates limitations known to exist in present density controllers. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.